Democratic Sen. Barbara Boxer of California told POLITICO that Hillary Clinton’s strong performance in the first Democratic presidential debate should be a warning to Joe Biden not to challenge her for the nomination.

“I just don’t think there’s a rationale for his campaign,” she told POLITICO California. “I think he should endorse Hillary … and go out that way.”

Boxer had previously urged Biden to base his decision on his own instincts.

The vice president has been mulling a potential run for weeks even as he’s publicly acknowledged he may not have the “emotional energy” to follow through with it after the May death of his son Beau to brain cancer. He’s been polling close to 20% of the vote in some surveys as Clinton has struggled amid controversy over her use of a private email server as secretary of State.

The image of “how strong she is,’’ said Boxer, makes a Biden candidacy less viable. “I just don’t see it.’’